As regulations on exhaust gases are stepped up in emission of combustion gases generated from heat engines, e.g., internal combustion engines, or combustion apparatus, e.g., boilers, through exhaust systems, improvements are performed on the incineration of engines, for example, compositions of fuels are improved. On the other hand, exhaust gases emitted from the above-described internal combustion engines and the like are cleaned by using exhaust gas treatment apparatus provided with filters and the like. In particular, with respect to diesel engines of automobiles, exhaust gas treatment apparatus provided with porous honeycomb filters are used in order to collect and remove particulate matter, e.g., soot, contained in exhaust gases.
As shown in FIG. 19, the above-described honeycomb filter has a honeycomb structure in which a plurality of cells 82 partitioned by partition walls 81 to serve as filter flow paths of an exhaust gas are included and these cells 82 are plugged alternately at an exhaust-gas-inlet-side end surface 83 and an exhaust-gas-outlet-side end surface 84. The exhaust gas is flowed into the honeycomb filter 80 through cells 82 opened at the exhaust-gas-inlet-side end surface 83, and is forced to pass through the partition walls 81 in the honeycomb filter 80, so that particulate matter in the exhaust gas is collected and removed.
When large amounts of particulate matter is deposited on the surface of the partition walls 81 of such a honeycomb filter 80, the pressure drop of the honeycomb filter 80 is increased, and the performance of an engine may be reduced since a back pressure is applied to the exhaust system on the engine side. Therefore, the particulate matter deposited on the surface of the partition walls 81 must be periodically removed to regenerate the above-described honeycomb filter 80.
Examples of methods for regenerating the honeycomb filter 80 can include a method in which the particulate matter is burnt off by using an electric heater, an afterburner, or the like. However, in this case, the honeycomb filter must be heated to at least 600° C. and, thereby, nonuniform temperature distribution tends to occur in the inside of the honeycomb filter 80 subjected to rapid temperature change or local heat generation. As a result, the honeycomb filter 80 may be damaged.
Consequently, an exhaust gas treatment apparatus has been proposed, in which NO contained in an exhaust gas flowing into the exhaust gas treatment apparatus is oxidized to NO2 having a high oxidation ability before the exhaust gas flows into the honeycomb filter 80, and combustible materials, e.g., soot, in the particulate matter deposited on the surface of the partition walls 81 of the honeycomb filter 80 are removed through oxidation by using the resulting NO2.
Specific example thereof can include an exhaust gas treatment apparatus in which an oxidation catalyst is placed upstream of the exhaust-gas-inlet-side end surface 83 of the honeycomb filter 80, NO contained in an exhaust gas of a diesel engine or the like is oxidized to NO2 by the above-described oxidation catalyst, and the honeycomb filter 80 can be regenerated by using the resulting NO2. Another example is an exhaust gas treatment apparatus having a configuration in which a plasma generation device is placed upstream of the exhaust-gas-inlet-side end surface 83 of the honeycomb filter 80 instead of the above-described oxidation catalyst.
However, with respect to the exhaust gas treatment apparatus in which the oxidation catalyst is placed upstream of the exhaust-gas-inlet-side end surface 83 of the honeycomb filter 80, the temperature required to activate the oxidation catalyst is high, about 400° C.-500° C. Consequently, there are problems in that, for example, when the diesel engine is operated under low-speed and light-load conditions, the temperature of the exhaust system is low. Therefore, the oxidation catalyst is not activated, and the honeycomb filter 80 cannot be regenerated. In the case where the above-described operation of the diesel engine under low-speed and light-load conditions is continued for a long time and large amounts of particulate matter is deposited on the surface of the partition walls of the honeycomb filter, when the catalyst reaches the activation temperature, the deposited particulate matter is oxidized and burnt at a time. Consequently, there is a problem in that the temperature in the exhaust gas treatment apparatus is increased rapidly and, thereby, the honeycomb filter in the inside is damaged due to a thermal stress.
With respect to the exhaust gas treatment apparatus in which the plasma generation device is placed upstream of the exhaust-gas-inlet-side end surface 83 of the honeycomb filter 80, nonthermal plasma is generated by the above-described plasma generation device and, thereby, NO contained in the exhaust gas can be oxidized to NO2 at a low temperature of 300° C. or less. However, there is a problem in that the exhaust gas treatment apparatus itself becomes too large to be equipped in an automobile or the like without constraints.
Furthermore, in the regeneration of the honeycomb filter, particulate matter can be effectively removed through oxidation when the exhaust gas containing NO2 produced through oxidation by nonthermal plasma is in an excited state. However, with respect to the above-described exhaust gas treatment apparatus, a problem occurs in that the excited state is reduced and the efficiency of regeneration of the honeycomb filter is deteriorated since there is some distance between the nonthermal plasma and the honeycomb filter.